Switchgear
Low voltage switchgear and LV switchboards
Main switchboards and power distribution assemblies up to 1000 V AC, sized from the rated current of the transformer that feeds them.
- Standard
- IEC 61439-1 and IEC 61439-2power switchgear assemblies up to 1000 V AC or 1500 V DC
- Incomer
- sized from the transformer currentcommonly 630 A to 6300 A
- Devices
- air and moulded case circuit breakersIEC 60947-2
- Internal separation
- Forms of separationset by the continuity of service required
- Short circuit rating
- kA for a stated durationset by the transformer and the upstream network
A main low voltage switchboard is sized from the transformer that feeds it. The transformer's rated current sets the incomer and the busbar. Its impedance and the upstream network set the prospective fault level, which the board must withstand and its breakers must interrupt.
The board is designed and verified to IEC 61439. The original manufacturer and the assembly manufacturer carry the verification of temperature rise, short circuit withstand and dielectric properties. The internal separation between busbars, functional units and terminals decides how much of the board stays energised while one part is worked on.
What gets specified
| Parameter | Typical options | What decides it |
|---|---|---|
| Rated current of the incomer | 630 A to 6300 A | The transformer rated current |
| Rated short time withstand current | kA for a stated duration | The prospective fault level |
| Form of internal separation | Form 1 to Form 4 as defined in IEC 61439-2 | The continuity of service required during maintenance |
| Incoming device | Air circuit breaker or moulded case circuit breaker | The current and the protection functions needed |
| Outgoing ways | Number, ratings and device types | The loads supplied |
| Busbar and connection | Cable or busbar trunking from the transformer | The current and the room layout |
| Metering and monitoring | Meters, power quality, communication | The owner's energy management |
| Degree of protection | IP rating | The room the board stands in |
Applicable standards
| Standard | What it covers |
|---|---|
| IEC 61439-1 | The general rules, including how design verification is split between original and assembly manufacturer |
| IEC 61439-2 | The board as a power switchgear assembly, up to 1000 V AC or 1500 V DC, including the forms of separation |
| IEC 60947-2 | The air and moulded case circuit breakers it is built from |
| IEC 60529 | The IP code of the enclosure |
Verification documents for an IEC 61439 board
| Document | What it proves |
|---|---|
| Design verification records | Temperature rise, short circuit withstand and dielectric properties of the design, from the original manufacturer |
| Routine verification report | The checks on the board as built, from the assembly manufacturer |
| Schedule of outgoing ways | Each way's device, rating and setting as built |
What a low voltage switchgear request needs
- Incomer rating, or the transformer rating and secondary voltage that feed the board
- Prospective fault level at the board, or the transformer impedance and upstream network data
- Form of internal separation required
- Schedule of outgoing ways with ratings and device types
- Connection from the transformer: cable or busbar trunking
- Metering, monitoring and communication requirements
- IP rating and the room dimensions
Questions buyers ask
What incomer rating does my transformer need?
Start from the transformer's rated current at the secondary voltage, 400 V or 433 V, then take the next standard device rating above it. The kVA to amps calculator gives that current from the rating and the voltage. Your designer confirms the choice against loading and discrimination.